Oscilloscope voltage calibrator



Dec. 30, 1958 A. B. KAUFMAN OSCILLOSCOPE VOLTAGE CALIBRATOR Filed April 6, 1954 SCILLSCPE VOLTAGE CALIBRATOR Alvin B. Kaufman, Los Angeles, Calif., assigner to Northrop Aircraft, Inc., Hawthorne, Calif., a corporation of California Application April 6, 1954, Serial No. 421,410

3 Claims. (Cl. 324-121) This invention relates to oscilloscope voltage calibrators, and more particularly to circuitry arrangements and components for a new and useful instrumentality for measuring electricalvalues of voltages when viewed on the screen of a cathode-ray tube associated with an oscillograph. l

With the advent of oscilloscope voltage calibrators,

' calibrator is that its referenced output should remain accurate over a wide range of A. C. line voltage variation.

'The circuitry described herein meets this requirement.

With this. calibrator, the line voltage may vary from 90 to 135 volts without any visible shift of the calibrating pattern on the oscilloscope.

All commercial calibrators are designed to produce a square wave at line frequency. Thus, ignoring any tilt of the pattern due to poor low frequency response in the oscilloscope, the peak-to-peak value of the pattern (at 60 C. P. S.) is used to determine the signal level.

Where the frequency of the signal to be calibrated is considerably above the line frequency, the oscilloscope gain may have fallen off, and the low frequency calibration may not be accurate unless a scope-frequency correction factor is employed. Furthermore, there is always the nuisance of adjusting the sweep frequency down from the test frequency to line frequency in order to secure a locked pattern. A

in order to calibrate at the same frequency as the signal to be calibrated, the invention described has provisions for injection of a signal frequency voltage, between l C. P. S. and 20,000 C. P. S., which sets the output frequency of the calibrator. An injected frequency Calibrating voltage level between approximately 6 and 120 volts R. M. S. is-required. The load on the signal source is l megohm and may in general be securedv from some section of the circuit under test, with negligible shunting or loading effect. 4

The circuitry of the device relies in part on the constent-current characteristics of a pentode with fixed screen excitation. Basicly, the circuitry consists of a pentode which is driven alternately between cutoff and plate current saturation. It is apparent from the plate current curve of a pentode that for a fixed bias the plate curthe measurement of the voltage amplitude of observed rent of the tube is substantially independent of plate voltage variation (if over 50-100 volts), and that a plate load resistor would develop an output voltage independent of everything except screenvoltage.

Since the plate current curves of a pentode tube' are almost, but not quite fiat, a variation of about 5% in the output voltage with change of plate voltage over the specified A. C. line voltage range must be corrected. Thus, it is necessary to regulate both the plate and screen voltage. Almost all voltage regulation systems employ a gaseous regulator tube either for direct control or as a reference control. As the current drawn by the pentode clipper is only a fewmilliamperes, there is no need for a complicated voltage regulator, a VR tube being directly applicable.

Accuracy of the voltage calibrator is directly proportional to the screen and plate voltage regulation. An OBZ VR tube has an operating voltage of volts, its regulation is l volt, or slightly better than 1%. Potentiometer loading error, however, causes the outputrpo tentiometer to be nonlinear by approximately 3%. This causes a possible over-al1 error of 4%. Potentiometer loading error occurs because the load resistor (the output multiplier) shunts the output resistance of the potentiometer sov that a given rotation no longer represents a proportional resistance and voltage output, and error is towards reduced output. To compensate for this error a simple mathematical correction may be made in the potentiometer dial reading when utmost accuracy is required, the equation being known to the art.

The practical application of the present invention for Calibrating the voltage amplitude of a signal in conjunction with an oscillograph, is well known to those who are familiar with the art.

It is the object of the present invention to provide a calibrator in which heater voltage variation does not have an appreciable effect on amplitude of Calibrating voltage as such variation causes only minor changes in the plate current saturation and cutoff points.

Briefly stated the invention comprises an oscilloscope Calibrating device vfor producing square Wave Calibrating voltages in which a constant peak-to-peak voltage output level is maintained at the frequency of the signal to be calibrated despite variations in voltages supplied thereto.

Other more specific objects and features of the invern tion will appear from the following detailed description, having reference to the accompanying drawings:

In the drawings:

Figure l is a diagram of the circuit of the invention.

Figure 2 is a block diagram of the application of the circuit of the invention to an oscillograph.

Referring to Figure 1, the electrical circuitry embodies a clipping tube 10 functioning as an overdriven voltage amplifier obtaining its grid drive voltage from a signal I i aplate. load resistenti, rlhe.. output network` is connected between the plate and cathode of tube l and consists of multiplier resistors 14, the output of each increment being selected by throw switch and quantitatively adjustable by variable resistor 16.

Bias for the control grid of tube 10 is obtained in part from the grid coupling resistor 17 and in part from the capacitance. which?.V becomesvcharged as-a result of grid current flow intube 10 vduringta portion'fo'fthe in'- put signal.

Throw switch 15,` when-thrownnto acertain position, connects the inputrsignal terminal 19- directly to the output terminal 2%; The source of positive voltage is obtained from a=trans former 2i havinga primaryy winding 22j and a l to l ratio secondary winding 23, one side-of` which is ccnnected to a selenium c ell rectiiier 24, and iiltered' by means of capacitor 2S.Y Aq., substantially steady positive voltage is `obtained byconnectinga voltage regulator tube 26, of proper voltagel regulating: characteristics, at f the plate and screen source of positive voltage.

The symbols and values as indicated in the circuitry diagram of Figure 1, are alpreferred form of the invention.

in determining, for example, the peak-to-peakvalue t to be calibrated is applied to the input terminals i9 and I* i90: of the calibrator 27 and a frequency sample of the signal voltage between 6 and 120 volts is applied to terminals 2S andZSrz. The outputterminals() and 26a are then connected to the vertical input terminalsl 29 and 29a of an oscillograph Sti-and the signal pattern observed on' the screen 31 of the cathode-raytube.

The signal pattern isadjusted to a marker: on the screen 31 and, subsequently, instead of the signal voitage, the calibration voltage is switched to the osciliograph, which is adjusted untilnthe calibration voltage pattern is tangent with the adjusted marking line.l Theamplitude of the calibration voltage as ready on theA increment and dial position of the calibrator then correspondsto the peak-to-peak value tobe determined.

While in order to comply. with the` statute, the invention has been described in language more or less specific as to structural features, it is tobe understood that the invention is not limited to-the specific features shown', but that the means andconstruction herein disclosed comprise a preferred form of putting theinvention into effect, and the invention is therefore claimedin any of its forms or modifications within the legitimate andvalid scope of the appended-claims.

What is claimed is:

l. An oscilloscope voltage calibrator, comprising: a pentode tube for operating as a limiter amplifier, means for operatively applyingv a constant-value source of D. C.

potential across the screen grid and cathode of said pentode, a resistance connectedbetween-the screen grid and anode of said pentode, inputcir'cuit means connected between the control grid `and cathode of saidpe'ntode, calibrating input terminal-means connected to said input circuit means for` applying an- A. C. signal whose peak-to-peak amplitude is to be measured to said input circuit means to provide a. substantially square wave calibrating output from saidlimit'er amplifier at a known peak-to-peak amplitude, a fixed-resistance output potentiometer' connected inanoutputcircuit between the anode and cathode ofls'aid pentode, the. continuously movable tap of said output potentiometer. cooperating with a calibrated dial and pointer, a. plurality ofseries attenuator resistances connected between said potentiometer tap and said cathode,..amultiple-contact selector switchhaving, the junctions `between-said'series ,attenuator' resistances connected to respective contacts thereof, the pole of said selector switch being connected to an output terminal of said voltage calibrator, anda signal input terminal of said voltage calibrator connected to another separate contact of said selector switch and adapted to have said A. C. signal to be measured connected thereto, whereby an oscilloscope connected to said output terminal can be used to measure the peak-to-peak amplitude of said A. C. signal at said signal input terminal by visual comparison with the calibrated variable flattened output voltage of the same frequency from said calibrator when said A. C. signal is also fed to said input circuit means of said calibrator at an amplitude above the threshold of limiting.

2. An oscilloscope voltage calibrator, comprising: a pentode tube for operating as a limiter ampliiier, means for operatively applying a constant-value source of D. C. potential across the screen grid and cathode of said pentode, a resistance connected between the screen grid and anode of said pentode, input circuit means connected between the control grid and cathode of said pentode, terminal means for applying an A. C. signal whose peak-topeakl amplitude is to be measured to said input circuit means, a fixed-resistance output potentiometer connected in an output circuit between the anode and cathode of said pentode, the movable-tap of said output potentiometer cooperating with a calibrated dial and pointer, a plurality of series attenuator resistances connected between said potentiometer tap and said cathode, a multiple-contact selector switch having the junctions between said series attenuator resistances connected to respective contacts thereof, the pole of said selector switch being connected to an output terminaln of said voltage calibrator, and a signal input terminal of said voltage calibrator connected to another separate contact of said selector switch and adapted to have said A. C.` signal connected thereto, whereby an oscilloscope connected to said output terminal can be used to measure the peak-to-peakV amplitude of said A. C. signal by visual comparison with the calibrated variable flattened output voltage of the same frequency from said calibratorwhen said A. C. signal is also fed to the grid circuit of said calibrator at an amplitude above the threshold of limiting, said calibrator being powered from normal A. C. power lines, and including separateselector means'for feeding a line frequency signal to the input of said pentode limiter to the exclusion of said A. C. signal whose amplitude measurement is desired, whereby comparisons of calibrator output can also be made between waveforms at signal frequency and at line frequency.

3. An oscilloscope voltage calibrator, comprising: limiter amplier means for producing a known constant peak-to-peak iiattened output wave having a frequency equal to that of an A. C. signal whose peak-to-peak amplitude is to be measured, a power transformer for operating said voltage calibrator from A. C. power at line frequency, a rectifier, filter, and voltage regulator means connected to a secondary winding of said power transformer to produce a constant source of D. C. potential for operation of said limiter amplilier means, an additional unrectified secondary winding on said transformer, said limiter amplifier means having an input circuit with Calibrating input terminals adapted to have the A. C. signal connected thereto, means connected to the output of said limiter means to provide calibrated voltage divisions of said outputwave, means for selectively and alternately connecting said A. C. signal to be measured and any of said voltagedivisions to an-output terminal for presentation on an oscilloscope, a vtwo-position selector switch having the pole thereof connected to the limiter side of saidinput circuit, one of said calibrating input terminals being operatively coupled to one position contact of said selector switch for applying saidA. C. signal to said limiter input circuit when said selector switch is in a iirst position, and vmeans for applying-A. C. voltage from said additional secondary winding' to `said limiter input circuit when said selector switch is in a second position, said signal and voltage fed to said limiter input circuit being from said limiter means can be obtained at A. C. signal frequency or at power line frequency as desired.

References Cited in the iile of this patent UNITED STATES PATENTS 2,235,173 Shepard Mar. 18, 1941 2,291,648 Rider et al. Aug. 4, 1942 10 2,482,803 Smith et al. Sept. 27, 1949 6 Jackson May 1, 1951 Carpentier Aug. 7, 1951 OTHER REFERENCES Article by Charles Markey and H. L. Polak, published in Electronics, November 1949, pages 193, 195 and 197. Copies available in Scientic Library and 324-130.

Encyclopedia on Cathode Ray Oscilloscopes and Their Uses (Rider), published by Rider, 1950; page 548, Fig. 15-68, and page 505, Fig. 14-16 relied upon. Copies available in Scientific Library and Div. 69. 

